Determining the molecular events that produce and regulate the release of neurotransmitters is one of the central goals of molecular neurobiology. Defining these events, and how they are altered with experience and in disease, will provide an understanding of normal processes such as learning and serve as the biological basis for developing new therapies for mental illness, neurological disorders and addiction. The work proposed here addresses the function of Synaptic Vesicle Protein 2 (SV2), a glycoprotein constituent of all neurotransmitter-containing (synaptic) vesicles. In the last funding period we demonstrated that Synaptic Vesicle Protein 2 (SV2) is an essential modulator of neurotransmission. Loss of SV2A reduces neurotransmission in hippocampal and cortical neurons, and reduces the size of the readily releasable pool of vesicles in chromaffin cells. Reduced neurotransmission is not due to morphological changes or to a decrease in morphologically "docked" synaptic vesicles. Together these results suggest that SV2 is a positive modulator of vesicle priming that acts after vesicle attachment at the plasma membrane. We propose to use SV2 knockout mice to determine SV2's molecular function in fast neurotransmission and to resolve differences in isoform functioning. Our specific aims are: 1. To determine how neurotransmission is altered in cultured hippocampal neurons from SV2 KOs. 2. To use exogeneous protein expression in hippocampal neurons to test hypotheses of SV2 function. 3. To test the hypothesis that SV2B plays a crucial role in ribbon synapse maintenance and/or functioning.